1. Field of the Invention
The present invention relates to a measuring apparatus and a measuring method to be employed for measuring a surface shape and a wavefront aberration of an object to be measured through use of light, and to a method of manufacturing an optical component by employing the measuring method.
2. Description of the Related Art
Conventionally, a measuring method using light has generally been employed for highly accurate measurement of a shape of an optical component and a wavefront aberration thereof. In a process of obtaining a highly accurate shape and an aspheric shape, it is necessary to measure the shape of the optical element, determine a difference from a designed shape, and to perform a correction process using data thus obtained. In recent years, there has been such a tendency that the accuracy of the shape increases and the aspheric amount of the aspheric optical element increases. Further, a free-form surface optical element has been used as well. As a measuring method for a surface shape and a wavefront aberration of such optical elements, a Shack-Hartmann method has been known (see U.S. Pat. No. 6,750,958 and Japanese Patent Application Laid-Open No. 2003-322587).
FIG. 12 is an explanatory view illustrating a schematic configuration of a surface shape measuring apparatus as an example of a conventional measuring apparatus. In FIG. 12, light emitted from a light source 1 is collimated. The collimated light having a plane wave 2 is deflected on a beam splitter 3, and enters an objective lens 4. The incident light having the plane wave 2 is condensed by the objective lens 4 into light having a spherical wave 5. The light having the spherical wave 5 passes through an aperture 6, and is reflected on a surface 7 to be measured. The light reflected on the surface 7 to be measured passes again through the aperture 6 and the objective lens 4 in a sequential manner, and is converted into light having a plane wave.
When the shape of the surface 7 to be measured deviates from a shape of a spherical surface, however, the wavefront of the light passing through the objective lens 4 becomes a wavefront 9 that deviates from the plane wavefront due to the difference from the spherical surface. The light having the wavefront 9 passes through the beam splitter 3, and is condensed by a microlens array 10. The condensed light is then detected as light spots by an image pickup element 11 such as a CCD image sensor.
A picked-up image, which is generated by the image pickup element 11 and contains positional information of the light spots, is received by a computer 13 via a frame grabber 12. The computer 13 extracts the light spots from the received picked-up image containing the light spots as an optical image, and calculates barycenter positions of the respective light spots.
The computer 13 compares the calculated barycenter positions of the light spots to reference positions of light spots, which are acquired in advance through use of a reference spherical wave. Then, the computer 13 determines a shape difference between the shape of the surface 7 to be measured and the shape of the reference spherical wave based on the movement amounts (deviation amounts) of the barycenter positions of the light spots from the reference positions. The movement amount of a single light spot corresponds to a difference between an inclination of the reference spherical wave and an inclination of a region of the surface 7 to be measured, which corresponds to a light spot obtained by condensing light through a single microlens. Thus, the shape error of the entire surface 7 to be measured from the spherical surface can be determined through integration over the whole microlenses.
Note that, when measuring a wavefront aberration of the object to be measured, the object to be measured is irradiated with light, and the light passing therethrough is condensed by the microlens array. Then, the positions of the light spots are detected, and the wavefront aberration of the object to be measured is determined based on the deviation amounts of the detected positions of the light spots from the reference positions.
In the case of measuring the surface shape of the object to be measured by employing the above-mentioned Shack-Hartmann method disclosed in U.S. Pat. No. 6,750,958 and Japanese Patent Application Laid-Open No. 2003-322587, however, when the object to be measured is made of a substance which allows measuring light to pass therethrough as in the case of a lens, light reflected from a rear surface of the object to be measured (undesirable light component) is generated in addition to the light reflected from the surface to be measured (signal light component). In the example of FIG. 12, light reflected from a rear surface 8 of the object to be measured is the undesirable light component. Further, in the case of measuring the wavefront aberration of the object to be measured by employing the above-mentioned Shack-Hartmann method disclosed in U.S. Pat. No. 6,750,958 and Japanese Patent Application Laid-Open No. 2003-322587, multiple reflected light resulting from multiple reflection occurring inside the object to be measured (undesirable light component) is generated in addition to the light passing directly through the object to be measured (signal light component).
When those signal light component and undesirable light component pass through the microlens array, signal light spots and undesirable light spots are formed on the image pickup element. Therefore, in the case where the computer calculates the deviation amounts of the signal light spots from the reference positions based on the acquired picked-up image, there is a problem in that the undesirable light spots overlap with the signal light spots and the accuracy of the position detection is therefore deteriorated.